Reference is first made to a pyroelectric material as used in the present invention. In general, a piezoelectric material may be classified into a pyroelectric material and a non-pyroelectric material, depending on the presence and absence of spontaneous polarization. The pyroelectric material may further be classified into a ferroelectric material and a non-ferroelectric material, depending on whether the direction or sense of the spontaneous polarization P.sub.s can be switched by application of an electrical field or not. Hence, a pyroelectrical material can be used without exception as a piezoelectric material, and some of the pyroelectric materials can be used as ferroelectric materials. As characteristic of the pyroelectric material, it shows a pyroelectric effect, that is, the property that electrical charges are produced on the surface upon heating a part of the material. This property of the material is utilized for application of the material in the field of infrared sensors and heat sensitive elements.
Pyroelectric materials usually exist as single crystals or polycrystals or a complex with other materials. In general, single crystals can be manufactured only with considerable equipment and material costs and prolonged time. Moreover, the single crystals of a larger or a desired size can not be produced easily. On the contrary, the polycrystal and complex materials have advantages over the single crystals in any of these respects, that is, equipment and material costs, manufacture time and capability to be manufactured to a large or desired size. It is to be noted that the polycrystal pyroelectric material is formed only of pyroelectric crystal material and hence exhibits more intense pyroelectric effect than the complex pyroelectric material.
According to known methods for manufacture of such polycrystal or complex pyroelectric material, crystal particles or powders of ferroelectric material in which the sense of spontaneous polarization can be switched rather easily, such as BaTiO.sub.3 particles, are baked to a solid layer, to which an electrical field is applied for orienting the spontaneous polarization P.sub.s substantially in the same direction (poling process). However, only ferroelectric material showing spontaneous polarization that can be switched easily in the sense or direction can be used as starting material, but neither the ferroelectric material showing spontaneous polarization that can not be switched easily in the sense or direction nor non-ferroelectric material can be used for the preparation of a polycrystal or complex pyroelectric material. In addition, the sense of spontaneous polarization can not be aligned perfectly when the poling operation is performed on the baked material, and the pyroelectric properties then obtained are usually inferior to those possessed by the single crystal pyroelectric material.
In contrast to the prior art, discussed above, the present invention contemplates providing a method for the preparation of a pyroelectric material wherein any material showing pyroelectric properties can be used for preparation of the polycrystal or complex pyroelectric material with substantially uniformly aligned sense or direction of the spontaneous polarization P.sub.s, or in other words, with the pyroelectrical axes aligned in the same direction or polarity.